1. Field of the Invention
The invention relates to protection against current leakage to ground, particularly in electrical devices such as appliances. In another aspect, the invention relates to detecting reverse polarity in electrical devices.
2. Description of the Related Art
In electrical load devices supplied through conductors from a power source there is a particular failure mode known as a ground fault where current flows between one of the xe2x80x9chotxe2x80x9d conductors and ground. A typical cause of such a fault is where water or moisture comes into contact with the electrical circuit. One common solution to the problems posed by ground faults is the use of shields. Typically, plastic shields are used to route water or moisture away from the electrical circuit, whether the device is a hand-held appliance such as a hair dryer or a stand-alone appliance such as a dishwasher. But the use of plastic shields often leads to other problems. For example, shields may complicate assembly and service of a device, leading to increased leaks and loose wires. Some shields may themselves present fire hazards, requiring the addition of fire retardant materials to the device. Material and labor costs associated with testing shielded devices may render the devices too costly to be competitive. Moreover, shields will protect against only leakage they have been designed for. Other changes in device design or usage can alter the path moisture may take to an electrical component.
Whereas shields attempt to protect against ground faults by preventing the conditions that would lead to a failure, various devices known as ground fault interrupters (GFI""s) offer protection regardless of the conditions leading to the failure. A GFI senses any minute leakage current flowing within a load device from a line xe2x80x9chotxe2x80x9d conductor to ground. When such leakage current is sensed, current to the load device is immediately interrupted, thereby avoiding a shock hazard. A power relay unique to the GFI typically does the actual interrupting. The specific way in which a ground fault condition is usually sensed is by employing a differential current transformer to detect a current imbalance in the power input lines. If the current flowing into the load does not exactly equal the current flowing out of the load, then it is presumed that some of the current is diverting to ground.
A GFI typically uses its own relay to interrupt or open the circuit supplying the load. The contacts for the relay itself, however, are normally closed, which leaves open the possibility that the GFI may receive a current that welds the contacts closed under certain conditions, such as an electrical storm. When this condition occurs, the GFI becomes non-functional, because the welding prevents the relay from disconnecting the power source when a fault would otherwise have been detected. Household circuits protected by a GFI that deliver current to an electrical device normally have test buttons to determine whether the GFI is functional or not. But many electrical devices are not connected to GFI protected household circuits. And even if they have built-in GFI protection, they normally lack any way of determining the functionality of the GFI, because the GFI operates on a separate circuit independent of the load circuit.
The problems noted in the prior art are solved by the present invention of incorporating a GFI into the load circuit of the electrical load device. A protective circuit according to the invention is adapted for an electrical load device operable from first and second conductors connected to a power source by way of a relay. The relay is operable from a control means, preferably a microprocessor. In the protective circuit, a ground fault interrupter capable of sensing any imbalance in current flowing through the first and second conductors and generating a signal indicative of that imbalance, sends such a signal directly to the load relay so that the relay deactivates the electrical load device upon receipt of the signal.
In one aspect of the invention, the control means monitors the signal from the ground fault interrupter and deactivates itself when a signal indicative of current imbalance is received. Also, an AND gate can be provided between the ground fault interrupter and the relay and between the control means and the relay, wherein the AND gate must receive signals from both the ground fault interrupter and the control means for the relay to be activated. Any break in signal from either the ground fault interrupter or the control means will deactivate the relay, thereby deactivating the electrical load device.
In one embodiment, an improvement according to the invention is incorporated into an appliance of the type having at least one electrical load device operable from first and second conductors connected to a power source by way of a relay. The relay is operable from a control means, and a ground fault interrupter is provided that senses current flow in the first and second conductors and interrupts current to the electrical load device when current flow between the first and second conductors is not balanced. The improvement provides for the ground fault interrupter to send a signal indicative of current balance between the first and second conductors directly to the relay that operates the electrical load device, whereby the relay is automatically deactivated when the signal indicates a current imbalance detected by the ground fault interrupter.
In one aspect an AND gate is disposed between the ground fault interrupter and the relay and between the control means and the relay, wherein the AND gate must receive signals from both the ground fault interrupter and the control means in order to operate the relay, and a break in signal from either the ground fault interrupter or the control means will deactivate the relay. Preferably, the control means is a microprocessor. Also, the ground fault interrupter can send a signal to the control means, so that the control means will be deactivated when the ground fault interrupter senses a current imbalance between the first and second conductors. Preferably, the relay is closed only when current is flowing to the electrical load device.
In another aspect of the invention, means are provided to protect the circuit or an appliance against reverse polarity of current flowing through the first and second conductors. Typically, the second conductor is neutral. The protection means can include a relay to enable breaking current flow through the second conductor in the event of reverse polarity. Alternately, protection is provided where the relay is a double throw, double pole type. Moreover, the protection means can be a flow detector circuit to determine the direction of current flow in one of the first and second conductors. In this case, the first conductor is typically a hot wire conductor and the flow detector circuit determines the direction of current flow in the first conductor.
The flow detector circuit can generate a polarity signal to the microprocessor when reverse polarity is detected in the current flow and the microprocessor can stop current flow upon receipt of the polarity signal. Preferably, the flow detector circuit will comprise a digital comparator that will compare voltage inputs from the first and second conductors, and generate a signal indicative of current flow.
In this aspect, an AND gate can be provided between the ground fault interrupter and the relay and between the flow detector circuit and the relay, wherein the AND gate must receive signals from both the ground fault interrupter and the flow detector circuit in order to operate the relay, and any break in signal from either the ground fault interrupter or the flow detector circuit will deactivate the relay. This way, the flow detector circuit can operate independently from the microprocessor.
In a typical application, the appliance can be a dishwasher and the electrical load devices in the dishwasher can be a vent fan, a motor, and a resistance heater.